Background: The most important diagnostic feature of the audiogram is the air-bone gap. Both false and missed air-bone gap lead to misdiagnosed. Acoustic radiation from the bone vibrator, especially at high frequency, causes hearing through the air-conduction reflects false air-bone gap. Materials and Methods: A total of 67 (134 ears) individuals with age ranged between 14 and 48 years (mean age: 36.1 years) were taken in this study. Who had external auditory canal clean and dry with intact tympanic membrane and the air-bone gap >10 dB in high frequencies (i.e., 2000 Hz, 3000 Hz, and 4000 Hz) were included in the study. All the clients who had air-bone gap >10 dB in the frequencies of 2000, 3000, and 4000 Hz were reassessed for their bone-conduction thresholds by closing the external ear canal with the patient's own index finger during testing. Study Design and Statistics: This was an experimental study where statistical analysis and ANOVA was done to compare between open ear canal and closed ear canal condition among frequencies. One-way repeated measure ANOVA along with Bonferroni comparison test was done separately for open ear canal and closed ear canal conditions. Results: Results revealed significant difference in air-bone gap (ABG) between two conditions which were increasingly more for higher frequencies, that is, 2000 Hz, 3000 Hz, and 4000 Hz compared to lower frequencies. Conclusion: Acoustic radiation is the sound energy that may be detected by air-conduction mechanisms which escape from a bone vibrator. When the external auditory meatus is occluded, the acoustic radiation phenomenon can be controlled or avoided, enabling bone measures at the higher frequencies, especially at 2000, 3000, and 4,000 Hz to be more accurate. Thus provides more accurate audiometric diagnosis which is most importantly based on ABG.

Audiometric tests are considered the most basic test to assess hearing sensitivity of an individual. The diagnosis, selection of treatment, and other diagnostic procedures are heavily based on the audiometric findings. Therefore, the audiologists must have full control over artifacts or errors and keenly aware of the importance of audiometric findings and take measures to ensure the accuracy of the results before sending them to other health professionals.

The most important diagnostic feature of the audiogram is the air-bone gap. By distinguishing between conductive and sensory–neural impairments, the air-bone gap profoundly influences the patient's care. Both false and missed air-bone gap lead to misdiagnosed. The former can lead to inappropriate medical or surgical treatment, and the later can cause harmful delays in medical intervention or management. It is important that audiologists take great care in determining if air-bone gaps are accurate. Patients with sensory-neural hearing loss and those with normal hearing do not exhibit air-bone gaps or if air-bone gaps appear that should not be exceed more than 10 dB. On an average, it is true; however, audiologists have to consider the variability of air- and bone-conduction thresholds. This variability of threshold measurements, audiologists should not expect that all air-bone gaps in these patients will be zero. In fact, that should be the exception, not the rule.

One clinical concern regarding bone-conduction threshold measurement is the acoustic radiation from the bone vibrator. This emitted acoustic energy conducted by air medium and enters into the external ear canal (EEC). This can be sufficiently intense to serve as an additional clue to respond to the stimuli when it is presented to the bone pathway. The bone-conduction thresholds of the patient can be corrupted, appearing to be better than they really are. Such finding would produce false air-bone gaps, especially at higher frequencies. The diagnosis of this audiometric finding can also be compromised.[1],[2]

A normal-hearing individual hears through the acoustic radiation of a bone vibrator at a level which is subjectively more intense than the energy produced by the vibrator. When this happens, the threshold will seem better than expected and a false air-bone gap will be recorded. The size of this gap will depend on the amount of acoustic radiation present.[3] Acoustic radiation from the bone vibrator, especially at high frequency, causes hearing through the air conduction because of the transmission of its energy to inside the external auditory canal (EAC) has been discussed by many authors.[3],[4],[5],[6],[7]

In Bell et al. (1980), Shipton et al., and Frank and Holmes (1981) where the acoustic radiation at different octave frequencies from 500 Hz to 4 KHz were measured at the entry point of the human EEC using Radioear bone vibrators placed on the mastoid with different bone vibrators (Radioear B-71, B-72, and B-70A BC). All these studies observed that the maximum acoustic radiation found at 4000 Hz, especially when using Radioear, B-71 and B-72 bone vibrators.[8],[9] In another study, where the experiment of acoustic radiation effect was done on 148 individuals with the mixed hearing loss with a mean age of 44.7 years by both air- and bone-conduction audiometry. The bone-conduction thresholds for the frequencies of 2 KHz, 3 KHz, and 4KHz were obtained in two different conditions, open EAC and closed EAC (the patient occluded the ear canal with his own index finger). In the later situation, it was observed that the air-bone gap changed in many of the individuals, which altered the interpretation of the results from the mixed to the sensory–neural type.[10]

Harkrider et al. measured the sound pressure level at EAC for the frequencies of 2 and 4 KHz, in 50 individuals by placing the bone vibrator (Radioear B-71) on the mastoid and on the forehead. A probe microphone was placed on the EAC, contralateral to the stimulated mastoid. They found that there are clinically significant air-bone gaps (>10 dB) because of the acoustic radiation.[2] The air-bone gap >10 dB at high frequency (especially at 3 z and 4 KHz) causes misinterpretation. So how can one explain such air-bone gap?

The present study aimed to assess the magnitude of air-bone gap due to acoustic radiation from the bone vibrator on frequencies of 2–4 KHz. This study also looks on its impact of acoustic radiation on diagnosis while pure tone audiometry testing.

Materials and Methods

This study was conducted at the Department of ENT, Pt.J.N.M. Medical College associated with Dr.B.R.A.M. hospital. Sixty-seven (134 ears) individuals were taken in this study with age ranged between 14 and 48 years (mean age: 36.1 years, standard deviation: 7.6 years).

Inclusion criteria

Those who had EAC clean and dry with intact tympanic membrane and no prior history of ear difficulty for the last 2 years and the air-bone gap >10 dB in high frequencies (i.e., 2000 Hz, 3000 Hz, and 4000 Hz) were included in the study.

Exclusion criteria

Those who had ear discharge, negative middle ear pressure >−50dapa and +50dapa and those clients who did not have air-conduction response in high frequencies (i.e., 2000 Hz, 3000 Hz, and 4000 Hz) and those with normal hearing sensitivity were excluded from the study.

Methodology

Initially, the otoscopic examination was performed for all cases. Tuning fork tests were performed at all fundamental series from 256 Hz to 1024 Hz and conventional pure tone audiometry (Interacoustic, AC40, Denmark) with TDH 39 earphone mounted with supra-aural cushions (MX-51/AR), and Radioear B-71 bone vibrator was performed using the modified Hughson-Westlake procedure.[11] The hearing thresholds at each octave frequencies from 250 Hz to 8000 Hz for air conduction and from 250 Hz to 4000 Hz for bone conduction were executed for each ear in sound-treated double chambered room with ambient noise level within permissible limits.[12] Calibration of the audiometer was performed according to the American National Standard Institute (ANSI) (2004). The American National Standard specification for Audiometer, ANSIS3.6-2004, New York: ANSI. GSI TympStar version 2 Immittance Audiometry (Grason-Stadler, A/S, Kongebakken 9, 2765 Smφrum, Denmark) was used for middle ear analyzing with a 226 Hz probe tone frequency.

All the clients who had air-bone gap >10 dB in the frequencies of 2000, 3000, and 4000 Hz were reassessed for their bone-conduction thresholds by closing the EEC with the patient's own index finger during testing. All data were collected for the air conduction in frequencies 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 3000 Hz, 4000 Hz, 6000 Hz, and 8000 Hz from both ears as well as bone-conduction threshold for the frequencies 500 Hz; 1000 Hz; 2000 Hz; 3000 Hz; and 4000 Hz for both ears with the EAC open and closed condition.

Statistical analysis

Data were submitted to a descriptive statistical analysis, and ANOVA was done to compare between open ear canal and closed ear canal condition among frequencies. One-way repeated measure ANOVA along with Bonferroni comparison test was done separately for open ear canal and closed ear canal conditions.

Results

On descriptive analysis for frequencies 2000 Hz, 3000 Hz, and 4000 Hz for two-test condition, that is, open canal and closed canal, the result [Table 1] revealed more difference for air-bone gap (ABG) in highest test frequency, that is, 4000 Hz compared to other two test frequencies, that is, 2000 and 3000 Hz. Furthermore, with an increase in frequency, there was an increase in ABG values has been observed. The Δ mean value suggests a huge difference in ABG for 2000 Hz versus 3000 Hz and 2000 Hz versus 4000 Hz. This value indicates that with increase in frequency there was more impact on ABG difference for two conditions (open and closed ear canal), that is, more toward high frequency as compared to relatively low frequency. [Table 1] also reveals no frequency impact on BC threshold in closed canal condition, as the sound presented to the BC vibrator has to travel through the BC mode as the AC mode has been closed. Thus, there is not much difference in ABG values among three frequencies, namely 2000 Hz versus 3000 Hz and 2000 Hz versus 4000 Hz.

[Table 2] reveals the details of ABG in two-test condition for frequency 2000 Hz alone. Table shows that the minimum ABG for the interest frequency was 0 dBHL and the maximum ABG was 20 dBHL. For open canal condition, ABG ranged from 5 to 20 dBHL, and for the later condition, that is, closed ear canal condition, the ABG ranged from 0 to 15 dBHL. For open canal condition, maximum ear (47.01%) depicted the ABG of 15 dBHL ABG. Only 2.98% which was minimum revealed for ABG of 5 dBHL. Whereas for closed canal maximum ears (50%) revealed 10 dBHL ABG followed by 41.8% who revealed 20 dBHL ABG and only 1.5% ears had shown 0 dBHL ABG which was minimum ABG observed in closed canal condition.

Table 2: Distribution of air-bone gap for the open and closed external auditory canal situations in 134 ears at 2000 Hz

[Table 3] reveals the details of ABG in two-test condition for frequency 3000 Hz alone. The table shows that the minimum ABG for the interest frequency was 5 dBHL and the maximum ABG was 35 dBHL. For open canal condition, ABG ranged from 15 to 35 dBHL, where for closed ear canal condition, the ABG ranged from 5 to 20 dBHL. For open canal condition, maximum ear (40.29%) depicted the ABG of 25 dBHL and only 4.48% which was minimum revealed for ABG of 15 dBHL. Whereas for the closed canal, maximum ears (44.03%) revealed 15 dBHL ABG and only 2.98% ears had shown 5 dBHL ABG.

Table 3: Distribution of air-bone gap on the open and closed external auditory canal situations in 134 ears at 3000 Hz

[Table 4] reveals the details of ABG in two-test condition for frequency 4000 Hz alone. Table shows that the minimum ABG for the interest frequency was 10 dBHL and the maximum ABG was 45 dBHL. For open canal condition, ABG ranged from 25 to 45 dBHL, and for closed canal condition, ABG ranged from 10 to 20 dBHL and which was lower in value compared to former condition. For open canal condition, maximum ear (43.28%) depicted the ABG of 35 dBHL and only 2.24% which was minimum revealed for ABG of 25 dBHL. Whereas for the closed canal, maximum ears (52.24%) revealed 10 dBHL ABG and only 5.22% ears had showed 20 dBHL ABG which was minimum in number.

Table 4: Distribution of air-bone gap on the open and closed external auditory canal situations in 134 ears at 4000 Hz

[Table 5] depicts the relative details of tested frequencies, that is, 2000 Hz, 3000 Hz, and 4000 Hz. The maximum range observed for 3000 Hz in open canal condition and 4000 Hz in closed canal condition, that is, 20 dBHL. For 4000 Hz, the overall ABG range was found to be in higher value compared to other two test frequencies.

In the present study, Radioear B-71 bone-conduction vibrator was used. Although bone-conduction vibrator such as B-72 and B-70A is commercially available in the market but was not used in the study because it has been shown experimentally that more acoustic radiation found with Radioear B-72 and B-70A as compared to Radioear B-71.[2]

Site of bone vibrator placement

In the present study, bone-conduction vibrator was placed on mastoid process as previous studies revealed experimentally that when bone-conduction vibrator is placed on the forehead, it increases test–retest reliability and accurate result is obtained than those obtained from mastoid placement because the frontal bone tissue is relatively homogenous as comparison to mastoid tissue.[13] Although reliability factor appears to favor the use of forehead placement, the greater disadvantage is the dynamic range of testing. Forehead placement requires more energy to get thresholds, thereby reducing the amount of maximum output achievable with the bone-conduction stimulation. In other words, the dynamic range is greater for mastoid than forehead placement.[14] Furthermore, ANSI standards for bone conduction are based on mastoid placement, and no headband arrangements for forehead placement are commercially available that produce the necessary static force recommended by ANSI. Thats why more than 90% of audiologist continues to use mastoid placement for bone-conduction measurement.[15] This all reason leads the authors of the present study to use the mastoid placement for measurement of acoustic radiation among different frequencies.

Frequency and acoustic radiation

From the present study, it has been observed that the mean value of ABG increased with increase in frequency, that is, from 2000 Hz to 4000 Hz due to acoustic radiation [Table 6]. This indicates the impact of acoustic radiation having more acoustic radiation in comparison to relative lower frequencies. Previous studies also agreed with the findings of the present study.[2],[8],[9],[15] Furthermore, there are studies which indicated disagreement with the present study. Momensohn-Santos et al. (2008), who found differences, showing that there was a worsening in the bone-conduction threshold, and there was a statistically significant difference in the frequencies of 3000 and 4000 Hz, as it happened in the studies led by Cili (2008), Momensohn Santos et al. (2008). Such findings confirmed the presence of acoustic radiation in the high frequencies as suggested by Dirks and Malmquist, (1969); Tonndorf (1972); Lightfoot (1979), Bell et al. (1980), Shipton et al. (1980), Frank and Holmes (1981), and Silman and Silverman (1991).

Acoustic radiation and threshold estimation

The literature on bone-conduction measures presents studies associated with: the technical limits of the bone vibrator; the pressure and placement of the vibrator on the mastoid; use of masking; the influence of the middle ear diseases; and the phenomena of occlusion and acoustic radiation.

Practice in clinical audiology leads us to question to what extent the results from the bone-conduction assessment of an individual is true or if it is the product of interference of actions or facts which occur during audiometry.[15]

It is possible to avoid the acoustic radiation phenomenon since there was a worsening in the bone threshold when the EAC was occluded. These findings prove that the energy escape from the bone vibrator could not reach the cochlea because the EAC was blocked, as suggested by Silman and Silverman (1997). Silman and Silverman (1997) commented that among the factors which produced artifacts on the bone vibration results are the effect of acoustic radiation.

A normal-hearing individual hears through the acoustic radiation of a bone vibrator at a level which is subjectively more intense than the energy produced by the vibrator. When this phenomenon happens, the bone-conduction measure will seem better than expected and a false acoustic gap will be recorded. The size of this gap will correspond to the amount of acoustic radiation present.[5] This increased air-bone gap leads to the interpretation of sensori–neural hearing loss as mixed hearing loss. This leads to misinterpretation of audiometric findings which is of great relevance in the diagnosis and further management for an individual.

Conclusion

Acoustic radiation is the sound energy that may be detected by air-conduction mechanisms which escape from a bone vibrator. This leads to an unreal air/bone gap in the high frequencies may result in an unreal bone-conduction threshold. The present study also depicts the presence of acoustic radiation which leads to increased air-borne gap with increasing frequencies. Furthermore, when the external auditory meatus is occluded, the acoustic radiation phenomenon can be controlled or avoided, enabling bone measures at the higher frequencies, especially at 2000, 3000, and 4000 Hz to be more accurate. The study further gives the direction to have special precautionary steps while evaluation of hearing threshold at higher frequencies.